The present invention relates to a system and method for the printing of substrates for use in food packaging and, more particularly, a flexographic printing system and method for applying and curing radiation curable inks to a flexible, heat shrinkable web.
In the food packaging art flexographic printing processes and apparatus have been employed for applying print media to a flexible web of, for example, plastic material which is thereafter used for packaging food products. The flexographic printing presses employed in such an application utilize a large central impression drum about which individual print stations are radially arrayed. Each of the print stations prints or lays down an individual color on the web. During the flexographic printing process it is necessary to dry the color laid down at a print station sufficiently before it reaches the next print station so as to prevent smearing or pick-off of the ink at the succeeding print station.
Heretofore, flexographic printing systems and methods employed solvent based ink systems or water based ink systems which allowed for the interstation drying to be accomplished by blowing hot air on the substrate or web being printed. There are a number of disadvantages associated with these known, systems and methods.
A major disadvantage associated with solvent based ink systems results from the fact that the solvents in the ink systems are evaporated from the inks during the ink drying process thereby releasing volatile organic chemicals into the atmosphere. Today there are increasing government regulations which require the reduction and eventually the total elimination of the emission of these volatile organic chemicals to the atmosphere.. In addition to the emissions problem noted above, there is an inherent explosive hazard associated with solvent ink printing systems which are heat dried. A third and particularly troubling problem associated with the food packaging art is the inherent shrink problem which results from heat curing solvent ink systems on heat shrinkable flexible webs which are used extensively in the food packaging art. In order to avoid shrinkage very long ovens must be employed to gradually dry the web.
Water based ink systems have been increasingly used in flexographic printing systems and methods in an effort to eliminate the emissions and ,explosive hazard problems associated with solvent based ink systems as noted above. Water based ink systems, however, are subject to hot air blowing for interstation drying during flexographic printing and, therefore, suffer from the problems associated with printing on heat shrinkable flexible webs.
Radiation curable ink systems have been used in the past in various printing systems. For example, in offset printing systems ink systems which are cured by ultraviolet (UV) radiation are known in the art. These radiation curable ink systems require heavy loading of the ink with photo-initiators to promote the final ink curing by ultraviolet radiation. Such an ink system is not suitable for printing flexible, heat shrinkable substrates for use in food packaging for the simple reason that the high loading of photo-initiators required to promote ink curing leads to high amounts of migratable or extractable monomers. The high amount of migratable or extractable monomers would fail to meet FDA requirements for packaging materials having incidental food contact. FDA requires less than 50 parts per billion migratable or extractable monomers as measured in FDA extraction tests. In addition to the problem associated with migratable or extractable monomers, photo-initiators are extremely expensive and thus the radiation curable inks used with ultraviolet radiation curable systems are costly. A further problem associated with ultraviolet (UV) radiation curable ink systems is the high level of energy input required to affect final curing of the ink system. Food packaging applications are often highly abusive applications and, therefore, high energy level input is required for final curing of these ink systems to a point where they can be successfully used on the outside surface of the package. When applying a UV curable ink system to a flexographic printing system further problems arise. The nature of the flexographic printing system which required a plurality of radially arrayed printing stations would require individual ultraviolet radiation systems to be incorporated between each printing station for curing the ink laid down at the printing station before printing in a successive printing station. In light of the high energy level input required by each of these ultraviolet curing and drying systems, energy costs for operating a flexographic printing system employing ultraviolet radiation curable inks does not appear to be commercially viable, particularly for heat shrinkable webs. In addition, high intensity UV lamps radiate about 50% of their energy as infrared energy which results in a heating of the central impression drum which must be disaffected.
Radiation curable ink systems which are cured by electron beam (EB) radiation are known in the prior art. These EB radiation curable ink systems however are not adaptable for use in flexographic printing systems in that the electron beam generators are extremely bulky in size and, therefore, are not suitable for interstation use in a flexographic printing system. In addition, the electron beam generators are extremely costly and, therefore, could not be economically used in a flexographic printing system which would require up to, for example, 8 generators in a single printing system.
Naturally, it would be highly desirable to provide a system and method for the printing of substrates for use in food packaging and, more particularly, a flexographic printing system and method for applying and curing inks to a flexible, heat shrinkable web which overcomes the problems associated with known printing systems as discussed above.